# std::sample

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class PopulationIt, class SampleIt, class Distance, class URBG > SampleIterator sample( PopulationIt first, PopulationIt last, SampleIt out, Distance n, URBG&& g ); |  | (since C++17)

Selects n elements from the sequence `[`first`,` last`)` (without replacement) such that each possible sample has equal probability of appearance, and writes those selected elements into the output iterator out. Random numbers are generated using the random number generator g.

If n is greater than the number of elements in the sequence, selects all elements in the sequence.

The algorithm is stable (preserves the relative order of the selected elements) only if `PopulationIt` meets the requirements of LegacyForwardIterator.

If the value type of first(until C++20)*first(since C++20) is not writable to out, the program is ill-formed.

If any of the following conditions is satisfied, the behavior is undefined:

- out is in `[`first`,` last`)`.
- `PopulationIt` does not meet the requirements of LegacyInputIterator.
- `SampleIt` does not meet the requirements of LegacyOutputIterator.
- All following conditions are satisfied:

`PopulationIt` does not meet the requirements of LegacyForwardIterator. | (until C++23)
`PopulationIt` does not model `forward_iterator`. | (since C++23)

  - `SampleIt` does not meet the requirements of LegacyRandomAccessIterator.

- Given the type `T` as std::remove_reference_t<URBG>, any of the following conditions is satisfied:

  - `T` does not meet the requirements of UniformRandomBitGenerator.

The return type of `T` is not convertible to `Distance`. | (until C++20)

### Parameters

first, last | - | pair of iterators forming the range from which to make the sampling (the population)
out | - | the output iterator where the samples are written
n | - | number of samples to make
g | - | the random number generator used as the source of randomness
Type requirements
- `Distance` must be an integer type.

### Return value

Returns a copy of out after the last sample that was output, that is, end of the sample range.

### Complexity

Linear in std::distance(first, last).

### Possible implementation

See the implementations in libstdc++, libc++ and MSVC STL.

### Notes

This function may implement selection sampling or reservoir sampling.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_sample` | `201603L` | (C++17) | `std::sample`

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <random>
    #include <string>

    int main()
    {
        std::string in {"ABCDEFGHIJK"}, out;
        std::sample(in.begin(), in.end(), std::back_inserter(out), 4,
                    std::mt19937 {std::random_device{}()});
        std::cout << "Four random letters out of " << in << " : " << out << '\n';
    }

Possible output:

    Four random letters out of ABCDEFGHIJK: EFGK

### See also

random_shuffleshuffle (until C++17)(C++11) | randomly re-orders elements in a range (function template)
ranges::sample (C++20) | selects N random elements from a sequence (algorithm function object)

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